Confine local file access to configured roots

[IlyaasK]

Summary

• add a shared resolver for paths constrained to a configured root
• prevent Obsidian search and writes from following paths outside the vault
• reject app sources and icons outside the app directory during validation and upload
• cover direct traversal and nested symlink cases with regression tests

Testing

• 166 tests passed, including the published generated protobuf package
• Python compilation check passed
• git diff --check passed